One question first, why do you want to change from postfix to sendmail? Postfix is easier to configure and supports the mailbox and maildir formats that sendmail uses, so i think there is no need to switch to sendmail for old accounts?

I have production server (Slackware 10) with sendmail running with about 200 mailusers.

Now i am configuiring a debian system with ISPConfig because it's great in Configuration because some Users want to change own pwds and add some aliases. Now i am doing this with Webmin.

I want to copy all the users and the mailboxes to the new system so that old users can get the mails. And if a User want to create new account he ads one with ISPConfig.
So will have some old mailboxes that can not be configuired with ISPConfig and then the new mailboxes that the Users will create themself.

I am confused . I must migrate the mailboxes without touching original User/Pwds.

Postfix uses the same mailbox format as Sendmail, so you can simply copy over your old mailboxes. Also copy the appropriate entries for these users from /etc/passwd and /etc/shadow and add their domains and email addresses to /etc/postfix/local-host-names and /etc/postfix/virtusertable. Run